Yes, the container runtime flags are per kubelet wise[1], so you can have a cluster with a mixture of runtimes. I can see the value you mentioned in testing and trying out the rkt+k8s. In fact we did something similar a while ago where we had Docker running in the master node, and rkt running on the minion node to narrow our testing scope.

[1] http://kubernetes.io/docs/getting-started-guides/rkt/#spin-u...